This invention relates to programmable logic array integrated circuit devices, and more particularly to the provision and use of spare circuits for performing the functions of defective circuits in such devices.
Programmable logic array integrated circuit devices with spare circuits for performing the functions of defective circuits are well known as shown, for example, by Cliff et al. U.S. Pat. No. 5,434,514. In the arrangement shown in the Cliff et al. patent an entire column--including all the logic modules in the column, the circuits which switch logic signals into and out of those logic modules, the vertical interconnection conductors of the column and the connections between those vertical conductors and the horizontal interconnection conductors of the device, and even the connections between the vertical conductors and the input/output pins served by those vertical conductors--is functionally replaced when there is a defect anywhere in the column. This strategy works well in many cases, but there are some types of defects in a column that will cause the circuitry of that column to continue to render the device unusable even if that column is functionally replaced. For example, certain types of defects in the programmable memory cells of a column may cause input multiplexers in the column (which normally bring signals from selected horizontal interconnection conductors into the logic modules of the column) to be erroneously programmed so that two or more inputs of a multiplexer are short circuited via the output of that multiplexer. This may improperly short circuit two or more horizontal interconnection conductors to one another, thereby rendering the device unusable despite the availability of circuitry for use in replacing the defective column circuitry.
In view of the foregoing, it is an object of this invention to improve the provision and use of spare circuits in programmable logic array integrated circuit devices.
It is another object of this invention to provide programmable logic array integrated circuit devices with spare circuits that can be used to replace defective circuits on the device, and at the same time to improve control of the programming of the memory cells that determine how the replaced circuits operate to minimize adverse effects of the replaced circuits on the remainder of the device.